Deicing aircraft engine propulsor blades is essential to prevent engine thrust loss. One type of deicing employs electrical heating coils within each of the blades. After a sheet of ice has been allowed to form on the blades, power is applied to the coils, thereby melting the ice which touches the blades (i.e. the ice which is at the contacting surface between the blade and the ice). Since the blades are attached to a rotating propulsor, centrifugal force causes the remaining, non-melted, portion of the ice to be thrown from the blades in a radially outward direction.
The coils are not continuously powered because the centrifugal force of the rotating propulsor would cause continuously heated water to migrate from the radially inward most portion of the blades to radially outward portions of the blades where the water could freeze. Instead, the coils are periodically deactivated in order to allow ice to form on the blades. Applying power to the coils causes the ice to be "shed" (i.e. thrown by the centrifugal force) from the blades. Shedding is a more efficient way to remove ice from propulsor blades because it requires cyclically heating only the radially inward most portion of the blades. Eliminating coils within the radially outward most portions of the blades and eliminating the need to have power applied to the coils continuously reduces the cost of the blades and the power consumed for deicing.
Power is supplied to the coils within the blades by a stationary mounted power source which is coupled to the rotating propulsor through slip rings. The use of slip rings adds to the cost and weight of the engine. Therefore, it is desirable to have an alternative scheme, which does not require electro-mechanical means for supplying remote power to the propulsor, for melting ice off the propulsor blades.
U.S. Pat. No. 4,365,131, entitled "Microwave Ice Prevention System" (Robert J. Hansman Jr.) discloses mounting a microwave transmitter on the non-rotating portion of an engine frame and using microwaves to heat the water on the blades before the water turns to ice. The microwave ice prevention system described in the '131 patent does not require the use of slip rings to supply power to the propulsor blades.
However, the advantages of shedding ice off the propulsor blades is lost with a system such as the one disclosed in the '131 patent. Allowing ice to form on the blades before applying power to the microwave transmitter will not result in the same shedding effect as does allowing ice to form on the blades before applying power to electric heating coils. In the case of the heating coils, the ice which touches the blade is melted in order to shed the ice. The microwave transmitter, however, will melt the outer layer of ice instead of the ice which touches the blade and therefore shedding will not occur.
Since with the '131 system contemplates heating the water continuously or nearly continuously, the water will tend to migrate to the radially outward most portions of the blades and freeze, just as in the case of continuously heating the blades with electric coils. Therefore, continuously heating the water on the blades with microwaves requires that the microwave transmitters irradiate the entirety of the blade surfaces. Heating the entirety of the blade surfaces continuously or nearly continuously tends to increase power consumption and make the transmitters more expensive initially and more expensive to maintain.